At least one bikeforums member had one and it broke.
Thread with pics of broken: Rinsten saddle spring

From an engineering standpoint, I don't think it's a very good idea. There's no dampening, so the rider looks to bounce up and down quite a bit. You can see this even in their promotional video. And holy cow, they are expensive! For the price, why not go for a more proven, less bouncy, and possibly even lighter weight, elastomer-based suspension seatpost?